2. Environmental exposure assessment
2.3 Model calculations
2.3.7 Elimination processes prior to the release to the environment
The situation with respect to waste water treatment at industrial installations is less clear. It may be assumed that many of the larger industrial installations are either connected to a municipal waste water treatment plant or have treatment facilities on site. In many cases, these treatment plants are not biological treatment plants but often physico-chemical treatment plants in which organic matter is flocculated by auxiliary agents e.g. by iron salts followed by a sedimentation process resulting in a reduction of organic matter measured as COD of about 25-50%.
In the present document, the above described situation is taken into account as follows:
• On a local scale, it is assumed that waste water will pass through a STP before being discharged into the environment. Nevertheless, for the largest PEClocal in surface water, it is necessary to determine an aquatic PEClocal assuming that no sewage treatment will take place. This value should be determined in addition to the normal PEC which assumes sewage treatment to flag for possible local problems (this PEC/PNEC ratio will not normally be used in risk characterisation).
The alternative/additional PEC can be used to explore the possibility of environmental impact in regions or industrial sectors where percentage connection to sewage works is currently low, so as to give indications to local authorities for means of possible local risk reductions. The PEC without considering a STP-treatment will not be used in the exposure assessment, unless the substance considered has a specific use category where direct discharge to water is widely practised;
• For a standard regional scale environment (definition see section 2.3.8.1) it is assumed that 70% of the waste water is treated in a biological STP and the remaining 30% released directly into surface waters (although mechanical treatment has some effect on eliminating organic matter, this is neglected because on the other hand stormwater overflows usually result in direct discharges to surface water even in the case of biological treatment. It is assumed that these two adverse effects compensate each other more or less with regard to the pollution of the environment).
The degree of removal in a waste water treatment plant is determined by the physico-chemical and biological properties of the substance (biodegradation, adsorption onto sludge, sedimentation of insoluble material, volatility) and the operating conditions of the plant. As the type and amount of data available on degree of removal may vary, the following order of preference should be considered:
1. Measured data in full scale STP
The percentage removal should preferably be based upon measured influent and effluent concentrations. As with measured data in the environment, the measured data from STPs should be assessed with respect to their adequacy and representativeness.
Consideration must be given to the fact that the effectiveness of elimination in treatment plants is quite variable and depends on operational conditions, such as retention time in the aeration tank, aeration intensity, influent concentration, age and adaptation of sludge, extent of utilisation, rain retention capacity, etc. The data may be used provided that certain minimum criteria have been met, e.g. the measurements have been carried out over a longer period of time. Furthermore, consideration should be given to the fact that removal may be due to stripping or adsorption (not degradation). In case no mass balance study has been performed, the percentage of transport to air or sludge should be estimated, e.g. by scaling the fractions to air and sludge from the tables in Appendix II to the measured removal.
Data from dedicated STPs should be used with caution. For example, when measured data are available for highly adapted STPs on sites producing high volume site-limited intermediates, these data should only be used for the assessment of this specific category of substances.
2. Simulation test data
Simulation testing is the examination of the potential of a substance to biodegrade in a laboratory system designated to represent either the activated sludge-based aerobic treatment stage of a waste water treatment plant or other environmental situations, for example a river.
So far only the waste water treatment process can be studied in the laboratory by agreed methods, e.g. the Coupled Units Test (OECD, 1981). Removability is determined by monitoring the changes in DOC (Dissolved Organic Carbon) and/or COD (Chemical Oxygen Demand).
The Coupled Units Test is not suitable for adsorptive, poorly water-soluble and volatile substances because it is an open test and is only based on DOC analysis. Since, in addition, it is possible that adsorptive or volatile metabolites may be formed during biological degradation, this test cannot differentiate between biological degradation and other elimination processes. Investigations with a closed vessel version of the Coupled Units Test using radioactively labelled chemicals have been performed which would allow a determination of the complete mass balance and would also be suitable for volatile or adsorptive substances. However, there is no international standard method available for this test.
There is insufficient information available on the applicability of elimination data from the laboratory test to the processes of a real sewage plant. The results can be extrapolated to degradation in the real environment only if the concentrations that were used in the test are in the same order of magnitude than the concentrations that are to be expected in the real environment. If this is not the case, extrapolation can seriously overestimate the degradation rates especially when the extrapolation goes from high to low concentrations. If concentrations are in the same order of magnitude then the results of these tests can be used quantitatively to estimate the degree of removal of substances in a mechanical-biological STP.
If a complete mass balance is determined, the fraction removed by adsorption and stripping should be used for the calculation of sludge and air concentrations. In case no mass balance study has been performed, the percentage of transport to air or sludge should be estimated for example by using the tables in Appendix II.
3. Modelling STP
If there are no measured data available, the degree of removal can be estimated by means of a waste water treatment model using log Kow (Koc or more specific partition coefficients can also be used; see section 2.3.5), Henry's Law constant and the results of biodegradation tests as input parameters. However, it should be remembered that the d i s t r i b u t i o n b e h a v i o u r of transformation products are not considered by this approach. It is proposed to use in the screening phase of exposure assessment a
revised version of the sewage treatment plant model SimpleTreat (Struijs et al., 1991). With SimpleTreat, the sewage treatment plant is modelled as an average size treatment plant based on aerobic degradation by active sludge, and consisting of 9 compartments (see Figure 4).
This model is a multi-compartment box model, calculating steady-state concentrations in a sewage treatment plant, consisting of a primary settler, an aeration tank and a liquid-solid separator. Depending on the test results for ready and/or inherent biodegradability of a substance, specific first order biodegradation rate constants are assigned to the compound.
An improved process formulation for volatilisation from the aeration tank, which is also applicable to semi-volatile substances (Mikkelsen, 1995), has been incorporated in the revised version.
For the purpose of modelling a STP, the rate constants presented in Table 4 have been derived from the biodegradation screening tests. The modelling results from SimpleTreat using these first-order rate constants of 0, 0.1, 0.3 and 1 h-1 are tabulated in Appendix II. It contains relative emission data pertaining to air, water, and sludge as a function of Henry's Law constant and log Kow for the different biodegradation categories, according to Table 4. If no specific measured biodegradation rate data are available for the particular substance, the tabulated values from Appendix II should be used.
Typical characteristics of the standard sewage treatment plant are given in Table 7. The amount of surplus sludge per inhabitant equivalent and the concentration of suspended matter in influent are taken from SimpleTreat (run at low loading rate).
Figure 4 Schematic design of the sewage treatment plant model SimpleTreat
These values are the same as applied to derive the tables in Appendix II. At a higher tier in the risk assessment process more specific information on the biodegradation behaviour of a chemical may be available. In order to take this information into account a modified version of the SimpleTreat model may be used. In this version the following scenario's are optional:
• temperature dependence of the biodegradation process;
• degradation kinetics according to the Monod equation;
• degradation of the chemical in the adsorbed phase;
• variation in the sludge retention time;
• not considering a primary settler.
Table 7 Standard characteristics of a municipal sewage treatment plant
Parameter Symbol Unit Value
Capacity of the local STP CAPACITYstp [eq] 10000
Capacity of the regional STP CAPACITYregstp [eq] 2.0.107
Capacity of the continental STP CAPACITYconstp [eq] 3.7.108
Amount of wastewater per inhabitant WASTEWinhab [l.d-1.eq-1] 200 Surplus sludge per inhabitant SURPLUSsludge [kg.d-1.eq-1] 0.011 Concentration susp. matter in influent SUSPCONCinf [kg.m-3] 0.45
Consultation of the tables in Appendix II gives the following input-output parameters:
Input
HENRY Henry's law constant [Pa.m3.mol-1] eq. (6)
Kow octanol-water partitioning coefficient [-] data set
kbiostp first-order rate constant for biodegradation in STP [d-1] Table 4
Output
Fstpair fraction of emission directed to air by STP [-]
Fstpwater fraction of emission directed to effluent by STP [-]
Fstpsludge fraction of emission directed to sludge by STP [-]
Calculation of the STP-influent concentration
For local scale assessments, it is assumed that one point source is releasing its waste water to one STP. The concentration in the influent of the STP, i.e. the untreated waste water, can be calculated from the local emission to waste water and the influent discharge of the STP. The influent discharge equals the effluent discharge.
EFFLUENT Elocal Clocal =
stp water inf
106
• (17)
Explanation of symbols:
Elocalwater local emission rate to (waste) water during episode [kg.d-1] eq. (2)
EFFLUENTstp effluent discharge rate of STP [l.d-1] eq. (18)
Clocalinf concentration in untreated waste water [mg.l-1]
Calculation of the STP-effluent concentration
The fraction of the chemical reaching the effluent of the STP is tabulated in Appendix II. The concentration of the effluent of the STP is given by the fraction to effluent and the concentration in untreated waste water as follows:
Clocal Fstp
Clocaleff= inf • water (18)
Explanation of symbols:
Clocalinf concentration in untreated waste water [mg.l-1] eq. (16)
Fstpwater fraction of emission directed to water by STP [-] App. II
Clocaleff concentration chemical in the STP-effluent [mg.l-1]
if no specific data are known, EFFLUENTstp should be based on an averaged waste water flow of 200 l per capita per day for a population of 10,000 inhabitants (see Table 7):
stp stp
EFFLUENT = CAPACITY • WASTEWinhab (19)
Explanation of symbols:
CAPACITYstp capacity of the STP [eq] Table 7
WASTEWinhab sewage flow per inhabitant [l.d-1.eq-1] Table 7 EFFLUENTstp effluent discharge rate of STP [l.d-1]
For calculating the PEC in surface water without sewage treatment, the fraction of the emission to waste water, directed to effluent (Fstpwater) should be set to 1. The fractions to air and sludge (Fstpair and Fstpsludge resp.) should be set to zero.
Calculation of the emission to air from the STP
The indirect emission from the STP to air is given by the fraction of the emission to waste water, directed to air:
air air water
Estp = Fstp Elocal• (20)
Explanation of symbols:
Fstpair fraction of the emission to air from STP [-] App. II
Elocalwater local emission rate to water during emission episode [kg.d-1] eq. (2) Estpair local emission to air from STP during emission episode [kg.d-1]
Calculation of the STP sludge concentration
The concentration in dry sewage sludge is calculated from the emission rate to water, the fraction of the emission sorbed to sludge and the rate of sewage sludge production:
sludge
sludge water
C = Fstp Elocal SLUDGERATE
• • 106
(21)
Explanation of symbols:
Elocalwater local emission rate to water during episode [kg.d-1] eq. (2)
Fstpsludge fraction of emission directed to sludge by STP [-] App. II
SLUDGERATE rate of sewage sludge production [kg.d-1] eq. (21)
Csludge concentration in dry sewage sludge [mg.kg-1]
The rate of sewage sludge production can be estimated from the outflows of primary and secondary sludge as follows:
CAPACITY udge
SURPLUSsl EFFLUENT +
SUSPCONC
=
SLUDGERATE • inf • stp • stp
3 2
Explanation of symbols:
SUSPCONCinf concentration of susp. matter in STP influent [kg.m-3] Table 7
EFFLUENTstp effluent discharge rate of STP [m3.d-1] eq. (18)
SURPLUSsludge surplus sludge per inhabitant equivalent [kg.d-1.eq-1] Table 7
CAPACITYstp capacity of the STP [eq] Table 7
SLUDGERATE rate of sewage sludge production [kg.d-1]
Anaerobic degradation may lead to a reduction of the substance concentration in sewage sludge during digestion. This is not yet taken into account.
Calculation of the STP concentration for evaluation of inhibition to micro-organisms
Some substances have an adverse impact on microbial activity. For the risk characterisation of a chemical upon micro-organisms in the STP, ideally the concentration in the aeration tank should be used. Assuming homogeneous mixing in the aeration tank, the dissolved (22)
PECstp = Clocaleff (23)
Explanation of symbols:
Clocaleff total concentration of chemical in STP effluent [mg.l-1] eq. (17)
PECstp PEC for micro-organisms in the STP [mg.l-1]
However, in the case of intermittent release, the concentration in influent of the STP is more representative:
PECstp = Clocalinf (24)
Explanation of symbols:
Clocalinf total concentration of chemical in STP influent [mg.l-1] eq. (16)
PECstp PEC for micro-organisms in the STP [mg.l-1]
The choice of using the effluent concentration is also reflected in the choice of the assessment factors used for deriving a PNEC for the STP micro-organisms. In modern waste water treatment plants with a denitrification stage, an additional tank is normally placed at the inlet of the biological stage. As the main biological degradation processes are taking place in the second stage, the microbial population in the denitrification tank is clearly exposed to higher concentrations of the substance as compared to the effluent concentration. As the technical standard of the STPs improves, this will have to be addressed in this assessment scheme in the near future.